The EndoCAb® IgM ELISA has been developed for determination of endotoxin core antibodies in human plasma or serum in patients or healthy individuals. Several studies show a consistent drop in postoperative levels of circulating anti-endotoxin core antibodies compared to the preoperative value. This drop has been interpreted as consumption of antibodies to endotoxin by systemic release of endotoxin. A hypothesis is that if the patients pre-operative endotoxin-core level is low, even moderately low, patients may not be able to cope with the efflux of endotoxin, which may have mild to severe clinical consequences. The assay is of interest for various experimental conditions ranging from in vitro LPS neutralization by plasma components to in vivo studies on kinetics of antibodies to endotoxin in health and diseases. The EndoCAb® standard median-units IgM (MU) are arbitrary and are based on medians of ranges for 1000 healthy adults in a particular locality. It has not been established whether normal endotoxin-core antibody values vary by region, culture or race. Users should establish appropriate local statistical controls for their studies. It is advised that studies of any patient group should always be controled by studies of appropriately selected contrasting clinical groups and/or healthy individuals recruited locally. EndoCAb® is a registered trademark. Used under license from the Scottish National Blood Transfusion Service.